THE FART-SECRET!

Owned by Christoph Wallnoefer
Sept 13, 2022
5 min read

What creates the Shift-Sounds, desperately wanted by many!

Besides the usual 0-60 and 60-120 times, there are little things discussed that much, as the distinct sound created by the more sportive gasoline (not Diesel!) vehicles during shifting. A lot of our support-tickets and social media comments somehow relate to the sounds (aka "shift-farts") created during shifts. Usually people are complainig about "not having them at all" or "my buddy's car being louder".  With this article, we want to do a bit of a deep-dive on this topic, so our customers can get a better understanding what's actually happening, whats causing that sounds and why they are there (or not there) in the first place.

First thing to know is, that those sounds aren't created on purpose, they are just a by-product of a specific way to reduce torque during a shift, without slowing down the turbos. In automatic transmissions shifts are always carried out as overlap shifts, which means the offgoing clutch is opened at the same time, the ongoing clutch is closed. During the ratio change phase of the shift, the on-going clutch has to overcome the torque of the engine to pull down the RPM's to the new gear. The more torque during this phase, the more pressure and friction is needed inside the transmission to shift to the next gear. As a consequence, the engine's torque needs to be lowered during the ratio shift phase, to help the transmission complete it's shift. The typical time of the ratio shift phase varies between 100 and 500ms, depending on how "sporty" the shift is. A faster shift has a shorter ratio-shift phase and vice-versa. It's this time-frame, where the transmission takes control over engine-torque and lowers it to an appropriate level for the actual shift carried out. Keep in mind, torque reduction is not a bad thing and on really fast shifts that phase is just 100ms (so 0.1 sec) long. Thats is a really short time-frame. Basically there are 3 ways to lower torque in a gasoline engine: 

  1. Close the throttle (or lower valve-lift in BMW valvetronic engines)

  2. Retard ignition

  3. Cut Ignition

Each method has it's advantages and dis-advantages, so in engine and transmission are maps stored to decide which method to use on a certain shift under certain conditions. For the purpose of this article, we can leave out 1) and 2) and focus on 3), which is the method of choice for quick shifts with as little boost loss as possible. Cutting ignition to reduce torque means, leaving throttle and injectors open, while voiding ignition on one or more cylinders.  As a result, the air-fuel mixture will not be ignited inside the cylinders, but pushed out to the turbos and ignited there.

The main advantages are 

  1. extremely quick reaction time, as there are no physical actuators to move like a throttle blade or wastegate (which would be slow) and

  2. the un-ignited mixture will keep the turbos pushing during the shift. 

The downsides are certain strain on the exhaust components, poor emissions and high vibrations.

That said you can't use this method everywhere. For instance using ignition-cut in low-RPM/high-load situations will create undesirable vibrations in the drivetrain. Using it on low-RPM/low-load (and therefore slow) shifts will lead to lots of unburnt fuel pushed through the engine, creating unnecessary heat on the exhaust side, poor emissions and also vibrations. Also it does not create any benefit in terms of controlling the shift in that RPM/load region. Cutting torque by ignition-cut is most desirable on fast, high-RPM and high-load shifts. This is where this method shines and does not only yield performance-advantages, but also creates the distinct bang sound during a shift. This sound is purely caused by igniting fuel outside of the engine in the exhaust side and is a by-product of carrying out a high-performance shift and never the actual goal. A transmission does not create "farts" on purpose.

It's just doing it's job, which creates some noise in the exhaust. Nevertheless, this sound has gotten very important to many people, which lead BMW to simulate it through your speakers on the recent G8X M3/M4, despite they are not using igntion cut. You will hear an artifical bang...but only in the cabin through your stereo. Due to some mis-configuration this can be even heard sometimes when shifting from 1-2 at a standstill. BMW will update their ASD software for sure. ;-) In general, most G-Series cars have this method turned off entirely, due to BMW no longer being focused on maximum performance on shifts. Recent setups tend to be setup more comfortable than in previous years. However, xHP Stage 2 and 3 maps will re-enable ignition-cut for those cars.

But even if ignition-cut shifting is used, it may be hard to detect for drivers or your fellow Instagram Fans. Recent exhaust setups have gotten very restrictive and now use multiple catalysts, particulate filters and mufflers to keep your car quiet on the outside. This is true for nearly all regions in the world, also including the U.S. market. As a result, on a vehicle with stock exhaust setup, or even with just a normal sport exhaust, the distinctive sound may be barely noticeable or it maybe less noticeable on a M340i than on a X3 M40i, due to different exhausts. It just vanishes through your cats and filters. In general the more free-flowing your exhaust is, the more audible things are going to be. Also cutting more torque will lead to more cylinders being turned off and therefore creating more "bang". But of course you don't want to remove all torque during a shift, just for creating sounds. During Launch Control or when activating Drag Mode in xHP, there won't be any torque reduction during shifts and therefore als no sound. (so using Drag Mode will not only put a lot of strain on your clutches, it will also remove any shift sound..Drag Racers shouldn't care, it will gain some time) 

However, xHP customers can use the "Torque Reduction" custom feature (on 8HP and DCT cars) to adjust things to some extent. (Note: 6HP vehicles in general do not support cylinder-cut method) So if you envy your buddy's car to be farting more, you may want to check your exhaust and/or play around with the custom setting. Or just enjoy your bang-on shifts. Because they are the same, whether you hear them through your exhaust or not.

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